1. Field of the Invention
The present invention relates to an illumination optical system, an exposure apparatus, and a method of manufacturing a device.
2. Description of the Related Art
In a lithography process, that is, a process of manufacturing, for example, a semiconductor device or a liquid crystal display device, an exposure apparatus transfers the pattern of an original (reticle or mask) onto a photosensitive substrate (for example, a wafer or glass plate having a resist layer formed on its surface) via a projection optical system. In recent years, as a projection exposure apparatus which transfers a pattern to, for example, a liquid crystal display device, an exposure apparatus which transfers a pattern having a larger area on a mask onto a substrate by full-field exposure is required. To meet this requirement, a step-and-scan type scanning exposure apparatus capable of obtaining a high resolution and transferring a large area has been proposed.
This scanning exposure apparatus transfers a pattern illuminated with a slit light beam onto a substrate by a scanning operation via a projection optical system. Such an apparatus that scans using arcuated light is available. In this type, a projection optical system is present between a first object having a pattern to be transferred, and a second object onto which the pattern is to be transferred, and an exposure process is performed in an arcuated exposure region using only a specific off-axis image point of the projection optical system.
At this time, to illuminate the first object with arcuated light, an illumination optical system described in Japanese Patent Publication No. 4-78002, for example, can be used. This illumination optical system employs a method of guiding a light beam emitted by a light source so that its cross-sectional shape becomes a rectangular shape at a position optically conjugate to that of the first object serving as a surface to be illuminated, and extracting an arcuated illumination distribution using an arcuated opening member from a region illuminated in a rectangular shape. However, the arcuated illumination distribution extraction scheme described in Japanese Patent Publication No. 4-78002 makes it impossible to use light other than the arcuated opening as exposure light. Hence, the light use efficiency is low, and it is difficult to increase the illuminance on the surface to be illuminated of the illumination optical system even when the output of the light source is raised.
Japanese Patent Laid-Open No. 3-171614 to be described next makes an attempt to optically couple a plurality of light pipes to convert the cross-sectional shape of a light beam emitted by a light source into an arcuated shape, thereby improving the light use efficiency. However, these days, to comply with a demand for improvements in throughput and imaging performance in a lithography process, an illumination optical system which more uniformly illuminates the first object plane at a higher illuminance is required. An illumination optical system described in Japanese Patent Laid-Open No. 3-171614 can uniformly illuminate the first object as long as only one light source is used. However, because this illumination optical system uses only one light source, it has a limit in achieving high-illuminance illumination. To achieve high-illuminance illumination using the illumination optical system described in Japanese Patent Laid-Open No. 3-171614, a plurality of light sources can be used. However, in this case, when, for example, the outputs of the light sources vary with time, and a difference in output therefore occurs between the light sources, a variation may occur in the illuminance distribution on the first object serving as a surface to be illuminated, thus making uniform illumination impossible. Also, Japanese Patent Laid-Open No. 3-171614 shows an embodiment in which an illuminance variation is reduced by increasing the dimension of each arcuated light pipe in the direction in which light travels, but it is difficult to manufacture such a long arcuated light pipe. Furthermore, when long light pipes are used, the size of the overall illumination optical system increases.
Japanese Patent Laid-Open No. 2000-164487 describes an illumination optical system which combines light beams, emitted by a plurality of light sources 1a and 1b, using prisms 8a and 8b and uniformizes the light intensity distribution of the combined light beam using a rod integrator 4, as shown in FIG. 16. The use of this illumination optical system makes it possible to uniformly illuminate a first object (a surface to be illuminated) 6 at a high illuminance using a plurality of light sources.
However, when the illumination optical system as described in Japanese Patent Laid-Open No. 2000-164487 is used, light incident on the rod integrator 4 has a nonuniform positional distribution. Hence, to achieve a uniform illuminance distribution on the first object 6 serving as a surface to be illuminated, the dimension of the rod integrator 4 in the direction in which light travels increases. As the rod integrator 4 gets longer, the size of the overall illumination optical system increases. Again, as the rod integrator 4 becomes longer, a larger light loss is generated upon internal absorption of the rod integrator 4 due to manufacturing errors resulting from factors such as the surface accuracy of the rod integrator 4 and the parallelism between its opposed surfaces, thus degrading the light use efficiency of the illumination optical system.